The present invention relates to rock crushing and handling.
As used herethroughout, the term “rock” or “rocks” refers, for example, to granite, limestone, gravel, top rock and other minerals that are to be crushed to form construction materials.
Traditionally, the production of crushed rock for use as construction materials has been performed by blasting away large rocks from the earth, and hauling the large rocks to a processing plant. At the plant, the incoming large rocks are passed through a primary or first-stage crusher that breaks the rocks into smaller pieces which then are further reduced in size by being passed through secondary, third, and at times fourth-stage crushers, such as gyratory cone and impact crushers for example.
After being passed through each of the crushers, the crushed rock is passed across a sizing device such as a screening apparatus that may comprise a plurality of screens with different size holes, in order to classify the rocks according to size. Rocks not passing through a screen may be routed for additional crushing. Rocks that pass through the sizing device are routed for final sorting. During the crushing, very fine material (“fines”) is produced which can adhere to the final product. Product gradation specifications often limit the amount of fine material to be included in the final product. Thus, open graded aggregates and some screenings and sand products are often washed to remove excess fines in order to meet product gradation specifications. The materials are then sold for construction materials.
A system for performing the above-described procedure is schematically depicted in FIG. 1 wherein incoming rocks are passed through a first-stage crusher 100 and then conveyed to a first screening apparatus 102. The smallest rocks passing completely through the first screening apparatus at 104 are collected for use as a commercial product, e.g., as road base material. Larger rocks not passing completely through the first screening apparatus are fed to a second-stage crusher 106 in which they are crushed to smaller sizes. Rocks crushed in the second stage crusher 106 are conveyed on a conveyor 107 to a second screening apparatus 108. Smaller rocks passing completely through the second screening apparatus at 110 are collected for use as a commercial product, e.g., as road base material. Medium sized rocks passing only partially through the second screening apparatus are conveyed to a third screening apparatus 112 which classifies those rocks into various sizes and conveys them to washing devices 114, 116, 118, and 119 where they are washed for use as product. Larger rocks are conveyed by a conveyor 120 to a third stage crusher 122 in which they are crushed to yet smaller sizes and returned by a conveyor 124 to the conveyor 107 which conveys them back to the second screening apparatus 108 for re-screening.
As the rock is processed to progressively smaller sizes through the crushing and sizing operations, quantities of very fine particles are generated. A high percentage of the production of these fines occurs in the third-stage crusher (and fourth-stage crusher, if used) where the smallest rocks are being formed.
As the rocks are being conveyed through the system, e.g., between the crushers and the sizing devices, it is necessary to spray the rocks with water by means of sprayers 126 in order to suppress the emission of dust by entraining fines within the sprayed water. Also, a low-volume air flow (e.g., 4,000 cfm) may be circulated around the rocks in order to collect dust generated as the rocks are conveyed.
It will also be appreciated that for many commercial uses, the final crushed rock product needs to be washed in order that adhered and excess fines can be removed therefrom before the rocks are shipped. This is true for both course graded and fine graded products. Therefore, there is a tendency for the final product to require vigorous washing.
The fines removed by washing and spraying are conveyed with the water to treatment facilities such as hydrocyclones, dewatering screens, thickeners and belt presses, or a settling pond 130 as shown in FIG. 1. The fines are then separated from the water which is re-used for washing and spraying. The fine material that is separated from the water has high moisture values, is typically difficult to handle, and has very limited uses. This material is, in most cases, either wasted on site or used as mass fill. It has little or no value, no consistency of gradation or moisture content and is generally a liability to the operation, adding to the expense and complexity of the operation.
It would, therefore, be desirable to provide methods and apparatus for producing crushed rock in a manner which minimizes the amount fine material that must be removed by washing and, in some instances, even eliminate the need for washing altogether.
It would also be desirable to produce crushed rock which is cleaner (i.e., carries less dust) and requires less vigorous washing operations.
It would also be desirable to enable fines produced during rock-crushing operations to be recovered in a state suitable for sale as a commercial product.